Conventionally, an electric motor drive apparatus includes multiple sets of windings corresponding to respective inverters. Hereinafter, a unit in which a winding set and the corresponding inverter are combined is also referred to as a system. Thus, the conventional electric motor drive apparatus includes multiple systems. In the conventional electric motor drive apparatus, when a fault occurs in one of the systems, which is also referred to as a faulty system, the faulty system stops operation. Then, another system, which is also referred to as a properly-operating system, starts operation in order to drive the electric motor. However, when another system is switched to operate when the fault occurs in one system, the fault occurred in the one system is not notified to a user. Thus, the user may continue to use the electric motor drive apparatus without any concern.
In an electric motor drive apparatus disclosed in JP 2012-25373 A, the fault occurred in one of the systems is notified to the user by the following configuration. In the electric motor drive apparatus, a vibration component having a predetermined frequency and a predetermined amplitude is added to a current command value related to a driving of the inverter included in the properly-operating system when the fault occurs in one of the systems. With this configuration, the user is notified of the fault occurred in one of the systems.
When applying the electric motor drive apparatus disclosed in JP 2012-25373 A to an electric power steering apparatus, the vibration component added to the torque may be perceived by the user as a fault occurred in the apparatus or as a discomfort feeling of steering a steering wheel. That is, the vibration component added to the torque may be differently perceived person by person. It is difficult to set the vibration component added to the torque so that the vibration component is surely perceived as the fault occurred in the apparatus to all of the users.